Clinical Assessment of Acid-Base Status: Comparison of the Henderson-Hasselbalch and Strong Ion Approaches

The traditional approach for clinically assessing acid-base status uses the Henderson-Hasselbalch equation to categorize 4 primary acid-base disturbances: respiratory addosis (increased P_CO2), respiratory alkalosis (decreased P_CO2), metabolic acidosis (decreased extracellular base excess or actual HCO^-₃ concentration), and metabolic alkalosis (increased extracellular base excess or actual HCO^-₃ concentration). The anion gap is calculated to detect unidentified anions in plasma. This approach works well clinically and is recommended for use whenever serum total protein, albumin, and phosphate concentrations are approximately normal. However, because the Henderson-Hasselbalch approach is more descriptive than mechanistic, when these concentrations are markedly abnormal the Henderson-Hasselbalch equation frequently provides erroneous information as to the cause of an acid-base disturbance. The new quantitive physicochemical approach to evaluating acid-base balance uses the simplified strong ion model to categorize 6 primary acid-base disturbances: respiratory acidosis (increased P_CO2), respiratory alkalosis (decreased P_CO2), strong ion acidosis (decreased strong ion difference), strong ion alkalosis (increased strong ion difference), nonvolatile buffer ion acidosis (increased plasma concentrations of albumin, globulins, or phosphate), and nonvolatile buffer ion alkalosis (decreased plasma concentrations of albumin, globulins, or phosphate). The strong ion gap is calculated to detect unidentified anions in plasma. The simplified strong ion approach works well clinically and is recommended for use whenever serum total protein, albumin, or phosphate concentrations are markedly abnormal. The simplified strong ion approach is mechanistic and is therefore well suited for describing the cause of any acid-base disturbance.